In a field of information technology, preventing leak of confidential information is of extreme importance. For example, a secret key used for data encryption is extremely important information, and a serious situation would result if it is stolen. The secret key is written and stored in, for example, a flash memory in a system LSI. Once written, a user usually cannot read out the secret key. However, there appeared some people who try to make unfair profits by unpacking an LSI package and detecting charge amount accumulated in a floating gate based on electric field distribution.
Japanese Laid-Open Patent Application JP-H07-219853 discloses a terminal device that can detect fraudulent access. The terminal device detects that a case is opened and then judges whether it is fraudulent disassembling by malicious third party or not. If it is determined to be fraudulent disassembling, the terminal device destroys confidential information stored in an installed EEPROM or SRAM.
The theft of confidential information is also possible in a case of an MRAM. The MRAM is a nonvolatile semiconductor memory that is promising in terms of high integration and high-speed operation. In the MRAM, a magnetoresistive element that exhibits “magnetoresistance effect” such as TMR (Tunnel MagnetoResistance) effect is used as a memory cell (refer to, for example, Japanese Laid-Open Patent Application JP-2005-86015 and JP-2006-134363).
FIG. 1 is a side view schematically showing a structure of a typical memory cell (magnetoresistive element) 110 of an MRAM. The memory cell 110 is provided with a magnetic pinned layer 120, a magnetic free layer 130 and a tunneling insulating layer 140. The tunneling insulating layer 140 is sandwiched between the magnetic pinned layer 120 and the magnetic free layer 130, and thus the magnetic pinned layer 120, the magnetic free layer 130 and the tunneling insulating layer 140 form an MTJ (Magnetic Tunnel Junction).
The magnetic free layer 130 is formed of ferromagnetic material and its magnetization direction is reversible. Although the magnetic pinned layer 120 also includes ferromagnetic material, its magnetization direction is fixed. More specifically, the magnetic pinned layer 120 has a synthetic ferrimagnetic structure consisting of a ferromagnetic film (first ferromagnetic film) 121, a ferromagnetic film (second ferromagnetic film) 122, a nonmagnetic film 123 and an antiferromagnetic film 124. The ferromagnetic film 122 is formed on the antiferromagnetic film 124, and the ferromagnetic film 121 is formed on the ferromagnetic film 122 through the nonmagnetic film 123. That is, the nonmagnetic film 123 is sandwiched between the ferromagnetic films 121 and 122. The ferromagnetic film 121 is in contact with the above-mentioned tunneling insulating layer 140. A magnetization direction of the ferromagnetic film 122 is fixed by the antiferromagnetic film 124. The ferromagnetic film 121 is antiferromagnetically coupled to the ferromagnetic film 122 across the nonmagnetic film 123 and thus its magnetization direction is fixed. Although the ferromagnetic film 121 and the ferromagnetic film 122 are opposite in the magnetization direction, their thicknesses are the same.
In the memory cell 110 thus constructed, two states are possible: an anti-parallel state in which magnetization directions of the magnetic free layer 130 and the ferromagnetic film 121 are anti-parallel to each other; and a parallel state in which the magnetization directions are parallel to each other. It is known that a resistance value (R+ΔR) of the MTJ in the anti-parallel state becomes larger than a resistance value (R) in the parallel state, due to the magnetoresistance effect. The memory cell 110 nonvolatilely stores data by utilizing the change in the resistance value. For example, the anti-parallel state is related to data “1” and the parallel state is related to data “0”.
Data rewriting is achieved by switching the magnetization direction of the magnetic free layer 130. For example, a write magnetic field generated by a predetermined write current is applied to the memory cell 110 and thereby the magnetization direction of the magnetic free layer 130 is reversed. Data reading is achieved based on magnitude of a tunneling current flowing through the MTJ. For example, in the case of data “1” (anti-parallel state), the resistance value of the MTJ is comparatively large and the tunneling current becomes comparatively small. On the other hand, in the case of data “0” (parallel state), the resistance value of the MTJ is comparatively small and the tunneling current becomes comparatively large. It is therefore possible to determine whether the data stored in the memory cell 110 is “1” or “0” by comparing the tunneling current with a predetermined threshold value.
In the case of the memory cell 110 of the MRAM, as described above, the data is determined by the magnetization direction of the magnetic free layer 130. Here, the magnetic free layer 130 generates a leak magnetic field, although it is weak. Therefore, as in the case of the above-mentioned flash memory, the data stored in the memory cells 110 can be externally read out by the use of a magnetometer. If important confidential information is stored in the memory cells 110, the confidential information may be stolen fraudulently.